Tooth reduction in the preparation for the fitting of a jacket crown involves circumferential reduction of the tooth which includes the creation of a subgingival shoulder which is uniformly recessed within the gingival sulcus. Correct fitting of the crown to the tooth requires the production of a dental impression of the tooth including its subgingival configuration.
Several methods are presently used to retract the gingival tissue disposed outwardly of its attachment to the tooth for access by the impression material into the gingival sulcus. In some cases tissue is removed by surgical means and in other instances this is achieved by chemically impregnated retraction strings, or cords, packed into the sulcus. In the former methods damage sometimes occurs to the underlying vascular and connective tissue which will prevent full regeneration of the gingiva. In the latter instances laceration of the tissue can occur and patient reaction to the chemicals is not always favourable.
It has already been proposed by U.S. Pat. No. 4,074,436 to secure in an impression tray inflexible tubes of copper, or the like, at the position of the reduced tooth or teeth. In that instance the distal end of the tube is pre-contoured to the gingival margin. After the tray filled with impression material, has been applied to the jaw of the patient further impression material is applied by a syringe down the copper tube, or tubes, to surround the reduced tooth and penetrate to its subgingival shoulder. The use of these tubes does not require retraction of the gingival tissue but, however, due to the difficulties in matching the contour of the distal end of the tubes to the gingival margin of the tooth and of precisely positioning the tubes in the tray, the services of a dental technician in his own laboratory are necessary. Resulting inconvenience to the dental surgeon and to the patient therefore occurs.
In U.S. Pat. No. 4,255,140 a solution to these problems is proposed by utilizing a flexible tube which is attached to the end of a syringe containing impression material. The outer end of the tube requires to be generally contoured to the gingival margin, which is selected from a stock, and is forced down into the gingival crevasse as impression material is exuded from the syringe as the tube is withdrawn from the tooth. A loaded impression tray is then applied over the deposited material about the tooth. Reliance is placed upon the longitudinal resiliency in the tube to enable its outer end to enter the gingival crevasse without injury to the gingival attachment. Two principal drawbacks have been found with this proposal, firstly the difficulty of the dentist readily obtaining a reasonably accurate contour on the tube, and secondly due to the flexibility of the tube it becomes far too floppy and unmanageable under pressurized conditions of use.